Recapped my mps-2500

[Eric D]

I would use these...

http://search.digikey.com/scripts/DkSea ... 054NPBF-ND

[interestingfellow]

Alright, i'm no EEE so i don't know nearly as much as most of ya'll. I know how to identify a transistor, resistor, etc. But with no block diagram or shema, I have very little idea which components are for what, so if you please, give a quick and dirty answer to the following:

The 12 fets (most of which I blew) are for the PWM power supply? The pwm output is smoothed by the 12 caps I already replaced?
The big blue caps are for smoothing out what?
The big toroid in the middle (off the bus bars from the power cables) is for filtering/noise?
What are the two other toroids for?
What are the 6 large green transistors? What are they for?
what are the other 6 big black transistors? What are they for?
What are the 4 smaller transistors mounted to the heatsink? And the two smallest transistors on the heatsink?
The smallest 2 components mounted to the heatsink are thermistors?
What is the 16 pin dip chip in the middleish of the board?
What are the enameled coils (with nothing in the middle) (physically near the short speaker out leads) for?
The two big (30a) fuses are for 12v protection of power in? The 4 smaller fuses are for the BJT's?

I'm gonna get this right, if it kills me.
The only failure is an attempt that was not ever made.

[valeks1]

Well there is your first problem, your amp needs like an 80A fuse tops, not 200A. So, if something were to go wrong (like it did), the 80A would hopefully blow well before the amp caught fire.

He got inside the amplifier 80A protection.

[Eric D]

Well there is your first problem, your amp needs like an 80A fuse tops, not 200A. So, if something were to go wrong (like it did), the 80A would hopefully blow well before the amp caught fire.

He got inside the amplifier 80A protection.

Actually he had dual 60A fuses installed in the amp, which IMO is way too much...

[interestingfellow]

my appologies,
I had used what was already in there. I wouldn't ever recommend replacing a stock component's fuses with larger ones (except emergencies or ASAP temporary). I couldn't see the rating on the those fuses becuase of the holders, and hate trying to pop them out unnecessarily; I had assumed they were 60's or 60 together.

I have high res/close up pics, but it's a beach to get them off my phone. I couldn't see any shorts, and I checked quite thoroughly. I can only assume I had damaged the fet's while beating on them.

I plan on replacing all those items I had to brute off. Good call on the rod trick to pop'm off, Eric. That is now logged away in the ol melon.

[Eric D]

-Power comes into your amp via those long buss bars.
-First it goes through the middle toroid which is called a "nulling toroid", and it helps filter noise on the line out.
-Power is split and goes through two big AGU fuses, one for each supply.
-It then goes to the 12 caps you installed, which are in two groups of 6 (6 for each power supply) (These caps will have 12-14V on them even when the car is off)
-The 2 16 pin devices you mention (maybe SQ2535s) send a PWM (pulse width modulated) signal to the resistors at the gates of the power supply FETs (the signal then feeds the FET's gates)
-The FETs, two groups of 4 (8 total) switch on and off rapidly from the SQ2535 controller. In the group of 4, there are 2 groups of 2, which oppose each other.
-The FETs outputs are shorted through the large toroids (2 of them), these are the power supply transformers.
-The switched 12V is boosted to maybe +35V, and -35V AC.
-The output of the transformer goes to the two diode packs mounted to the heatsink. SF302 and SF302A I think they are. Each group of for one supply.
-These diodes rectify the output of the transformer from AC to DC, and you get your 35V or 25V or what ever they are rail voltages.
-The big blue caps sit on these rails and keep ripple down (giving you smooth DC).
-Then you have the small fuses for the rails.
-After the fuses are the lines of thick copper to feed current to the output devices.
-3 green ones do one half of the signal, the 3 black ones do the other half of the signal.
-Same for the other side, so you have 12 output BJTs in all.
-Output from these goes through some large resistors, and then on to those big open coils. The amp would work without the big coils, but they help with the amps ability to control the speaker some.

These amps are "triple darlington" So on the board are some smaller transistors sticking up. 2 of them are the voltage gain for the amp. The next 2 are stage one current gain, then you have the two smaller transistors mounted to the heatsink these are stage 2 of current gain, then you have the 6 outputs or stage 3 of current gain. On the bigger MS amps some of those on the board need little heatsinks clipped on them, as they generate a lot of heat on their own.

If you split the amp down the middle (parallel to those big buss bars), and divide the amp in half, each side is a dedicated mono amplifier, with its own filtration, power supply, rail filtration, and output sections. Only in a few spots for the signals of the preamp section share components between the sides of the amp.

[Eric D]

The smallest transistors mounted to the heatsink between the other small transistors are for bias control.

Your amp is Class A/B which means it has some Class A bias current flowing through it while idle. These little transistors are involved in this. I believe the goal of mounting them to the heatsink is to keep them at the same temp as the output transistors are, so the bias setting remains stable. If these little transistors were on their own, the bias could "run away" at a certain temp, thus destroying the amp.

I have done a lot of experimenting with PG amps over the years as I repair them, and I have found they sound the same with the bias all the way down, which should limit them to Class B. However, I have done all this listening on 8 ohm or maybe 4 ohm speakers, and I would guess you will only hear a difference when you load the amp down more. These amps have enough feedback to keep the output clean with 4 or more ohm loads and no bias current.

[interestingfellow]

Thank you, Eric!

In the even that I don't get the schema for my repair guy to look at, that description should be good enough. Actually, I know these amps are actually fairly simple as PCB's go, so with that description, any EEE101 student would be able to figure it out.

I often live in a daydream world, where I have lots of time, and enough money to screw around with all the things I'm interested in. That being said, after having seen how "simple" the guts are, I had immidiately thought "I wonder if I could build it from scratch? it couldn't be more than $120 in parts."

Back to reality....

[00goobs]

I think the power supply side is pretty near standard, but that voltage/amperage gain stage actually takes a bit of experience to understand for me. I was very lucky to have a working 2125 to compare readings to and components with while repairing a damaged 2125.

[Eric D]

The beauty of these amps for most repairs (where one channel is bad) is they are mirror images of each other internally. If part 114 is bad, then you can look at part 214 on the other side of the amp to know what the good part is.

[interestingfellow]

Aight!
I had written a htm page, but can't attatch it. Here is the link to the photobucket folder with all the pics I took while stripping it apart. Best I can do. Take a look, and lemme know, please.


http://s632.photobucket.com/albums/uu44 ... ?start=all

[Eric D]

I don't think your board ended up with much damage at all. Remove all the bad transistors (8 of them), and replace them and the resistors which drive their gates.

[interestingfellow]

I'm thinking I should replace all the Fets, Bjt's, and similar items that had thermal pads holding them to the heatsink, as I really feel like that's why the other fets failed (physical damage from beating them off). I can't get a schema for the board, buy I'm still gonna try to get my guy to look it over once and see what he sees.

Should I be concerned about (or some of) those new caps? A couple of them got sprayed with a little fire which left some carbon on the shrink wrap(s).
Stupid question: which resistors are driving the gates on those? (I assume the 1/4 watt size, coming right off the leg of the fet, like R329 as seen here http://i632.photobucket.com/albums/uu44 ... AG0308.jpg)?
I'd rather ask and look like an idiot, than screw up and prove it!





Huh, huhuh, I said beating off...

[Eric D]

The resistors are as close to the FETs as you can get, so I think you found the correct ones. They are greenish in color. You should replace them with fusable types as the originals are fusable, and that is a good thing.

You might not be able to replace the SF302, and SF302A parts. Those are pretty rare, and I have not found an exact replacement, but I have not really looked too hard either. I am sure something is out there somewhere.

[interestingfellow]

I would use these...

http://search.digikey.com/scripts/DkSea ... 054NPBF-ND

I thought you were specifying those for the sf302 replacement? Yes/No?

[oldschoolfan]

I would use these...

http://search.digikey.com/scripts/DkSea ... 054NPBF-ND

I thought you were specifying those for the sf302 replacement? Yes/No?

No, those are the mosfets for the power supply, quantity 8, to replace the IRFP 044's that you probably had in there.

I used the ones listed in the link and they work great for the application.

[interestingfellow]

Silly question:

would upgrading to higher rated (amps) fets and bjt's increase WRMS?

I probably shouldn't even be thinking bout it.

[oldschoolfan]

Silly question:

would upgrading to higher rated (amps) fets and bjt's increase WRMS?

I probably shouldn't even be thinking bout it.

My knowledge on this is limited. I do know that upgrading the power supply fets, which the 054's are an upgrade, will not increase anything. They are controlled by other circuitry on the board. They just wont be working as hard within their maximum capabilities to do what the amp asks of them. On the BJT's, the output side, I know nothing.

[Eric D]

The BJTs would work the same, as far as upgrading them won't really get you anything more out of the amp. The voltage the power supply runs at, and the current it can provide are what determine the output of the amplifier (for the most part).

To upgrade an amplifier you need to rewind the transformer for higher voltage, and then adjust the feedback to the PWM controller to make use of the additional voltage. Otherwise if you just change the transformer, the PWM controller will back off the output of the power supply to meet what the amp did before, at the expense of efficiency.

[interestingfellow]

Riiight.

I'll upgarde for quality, not power....

Thanks again!

[interestingfellow]

OK,
Life has gotten in the way of living, and I've finally gotten around to fixing this dern thing.
I cleaned it up as best I could with %91 isopropyl, and started reading components with my dmm.

Looking at the attached photo:
the green boxes are the general area where the fusible resistors are??? I hope so, cause all those suckers had a reading of nada/0/zilch. Where do I get those from?

The red boxes are other resistors (that I had originally thought were the fusible resistors for the fet gate) that DMM out around 55ohms.

I also checked many other resistors for open condition, but haven't had time to find all the band values on them. I just checked most of'm against each other.

The yellow and blue boxes are diodes. My DMM set to the diode check, checking the yellow box diodes, it reads 002 both ways on both diodes. I checked the blue box diodes for a comparison, I get a reading of 500ish one way and 1200ish the other way (ohms?).

I can't thank all of you enough for all the help.
But I really feel in over my head here.

[ttocs]

You would need to remove teh diodes to check them correctly I think.

[00goobs]

I believe all the little blue resistors are fuse resistors that are near the transistors. 55ohms seems right as they are 100ohms fuse resistors for the power supply fets and are 2 in parallel in the circuit. You should lift a leg on the fuse resistors to really test them, but 55ohms does sound right for 2 100ohm resistors in parallel.

Even in circuit, I think the diodes on the power supply should only show continuity one way (Yellow square). If you have a reading both ways, they should be replaced. I think you should have 0.0xxohms for these diodes as well as I believe they are supposed to short directly to ground in case someone were to reverse the + and - on the power inputs and save the fets from blowing up....

Your green square resistors are the drop down for the audio input +/- 15 volts for the opamps. I don't think they are fusible and need to be replaced with 470ohm 1 watt if you don't have a reading accross them.